1. Field of the Invention
The present invention relates generally to an electric power supply system in which a power converter such as an inverter or the like and other AC power as an electric generator can be operated in parallel by maintaining a current-balanced state. More particularly, the invention is concerned with an electric power supply system in which a power converter such as an inverter and an AC power source such as a generator can be changed over with each other without involving interruption of power supply to a load by suppressing a voltage build-up due to a cross current.
2. Description of the Related Art
A method of operating in parallel a power converter such as an inverter and another AC power source such as a generator by imparting a drooping characteristic to an output frequency of the inverter is well known in the art.
The power supply system of this type is employed, for example, as a static-type power source system for an aircraft. More specifically, during an ordinary flight operation, an onboard electric generator is used as a power source for supplying electric power to a load, while after the landing, operation of the onboard generator is stopped and the power supply to the load is changed over to an inverter installed on the ground. At that time, two power sources are temporarily connected in parallel to the load, whereon these power sources are changed over without interrupting the power supply to the load.
For a better understanding of the present invention, a power supply system of this type known heretofore will be discussed in some detail. FIG. 8 is a block diagram showing schematically a structure of a hitherto known power supply system which includes a power converter connected such that it can be operated in parallel with a generator. Parenthetically, this type power supply system is disclosed, for example, in Japanese Patent Publication No. 24690/1976.
Referring to the figure, a power converter typified by an inverter 1 serves to convert a DC power P.sub.d (a product of a DC voltage V.sub.d and a DC current I.sub.d) supplied from a DC power source (not shown) to an AC power P.sub.1 (a product of an AC voltage V.sub.1 and an AC current I.sub.1). Another AC power source such as an electric generator 2 is provided in parallel to the inverter 1.
The output frequency (frequency of the AC power P.sub.1) of the inverter 1 is controlled or regulated in dependence on the voltage by an inverter control circuit described hereinafter so as to be substantially equal to that of the output frequency of the AC power source 2 (i.e., the frequency of the AC power P.sub.2).
The inverter 1 and the AC power source 2 which constitute a parallel-type power source are connected to an AC bus 5 via switches 3 and 4, respectively. Further, a load 6 is connected to the bus 5 so that the AC power P.sub.1 or P.sub.2 generated by the inverter 1 or the AC power source 2 can be supplied to the load 6. In the parallel operation, both the switches 3 and 4 are closed, and thus the inverter 1 and the AC power source 2 share the power supply to the load 6 with each other. In the ordinary or normal operation, either one of the switch 3 or switch 4 is closed, whereby either one of the inverter 1 or the AC power source 2 bears the power supply to the load 6.
An inverter control circuit for adjusting or regulating the output frequency of the inverter in accordance with an effective or active component of the AC power P.sub.1 is comprised of a voltage control circuit 10 for controlling the AC voltage V.sub.1 supplied from the inverter 1 so that it coincides with a reference voltage V.sub.ref, a subtractor 11 for determining arithmetically a deviation or difference .DELTA.V between the AC voltage V.sub.1 and the reference voltage V.sub.ref, and a voltage sensor 12 for detecting the AC voltage V.sub.1, wherein the elements mentioned above constitute a feedback control circuit for equalizing the AC voltage V.sub.1 outputted from the inverter 1 to the reference voltage V.sub.ref.
The reference voltage V.sub.ref is generated by a reference voltage generating circuit 13 which serves for determining the output frequency of the inverter 1 on the basis of an oscillation frequency f outputted from a voltage-controlled oscillator 14.
The effective power P of the AC power P.sub.1 output from the inverter 1 is detected by a power sensor 15, wherein the effective power P as detected is input to the voltage-controlled oscillator 14.
The voltage-controlled oscillator 14 exhibits a voltage-versus-frequency (P-f) relationship as represented by a solid line curve shown in FIG. 9. Incidentally, a broken line curve represents a power-versus-frequency (P-f) relationship of the AC power source 2 or generator.
The output frequency f of the voltage-controlled oscillator 14 lies at a center frequency f.sub.0 when the inverter 1 is in the no-load state (i.e., when P=0). However, this output frequency f droops as the effective power P increases. By way of example, when the effective power P output from the inverter i increases to P.sub.1 (&gt;0), the oscillation frequency f droops by .DELTA.f. On the contrary, when the effective power P is negative, i.e., when the AC power P.sub.2 is not consumed by the load 6 but fed back as a regenerative power, the oscillation frequency f rises.
FIG. 10 is a block diagram showing an exemplary configuration of the voltage-controlled oscillator 14 exhibiting the operation characteristics illustrated in FIG. 9.
As can be seen from this figure, the voltage-controlled oscillator 14 is comprised of oscillation circuitry 21 including a quartz oscillator which oscillates at a constant frequency F, frequency divider circuitry 22 for dividing the oscillation frequency of the oscillator circuitry 21 to thereby output, for example, an eight-bit signal 22a, an analogue-to-digital (A/D) converter 23 for converting an analogue signal output from the power sensor 15, i.e., the effective power P, into a digital signal such as, for example, a four-bit signal 23a.
The four-bit signal 23a and the eight-bit signal 22a are input to a digital multiplier 24 which serves to modulate the four less significant bits of the eight-bit signal 22a with the four-bit signal 23a to thereby regulate the constant oscillation frequency F in dependence on the effective power P. The output of the multiplier 24 is supplied to a frequency divider 25 which serves for dividing the frequency output of the multiplier 24 to thereby output a proper oscillation frequency f which corresponds to the output frequency of the inverter 1.
Next, referring to FIG. 9, description will be directed to operation of the known power supply system shown in FIG. 8.
It is first assumed that the switch 3 is closed with the switch 4 being opened. In this case, the output power P.sub.1 of the inverter 1 is supplied to the load 6 as a load power P.sub..delta.. Starting from this state, it is again assumed that the power supply to the load 6 is to be changed over from the inverter 1 to the AC power source 2.
For realizing the change-over of the power sources mentioned above without interrupting the power supply to the load 6 through the bus 5, the switch 4 is first closed to establish a parallel operation state in which the invertor 1 and the power source 2 are operated in parallel. Thereafter, the switch 3 is opened. In the parallel operation state, a cross current will flow between the inverter 1 and the AC power source 2, which current has a magnitude corresponding to differences between the output voltages and phases of the inverter 1 and the AC power source 2.
In this conjunction, let's assume that the voltage amplitudes of the output powers of the inverter and the AC power source 2 are equal to each other and that impedance of the inverter and the AC power source connected in parallel is provided only by reactance due to an internal reactance of the electric generator constituting the AC power source. On these assumptions, the cross current may be considered to be provided only by the active current component which is ascribable to the difference in phase between the output of the inverter and that of the power source, Consequently, in the parallel operation of the power supply system, the burden for power supply to the load is shared between the inverter and the power source or generator in dependence on the power-versus-frequency characteristics thereof.
In other words, the power supply system operates at such an operation point where the inverter 1 bears the supply of the power P.sub.1 with the power source bearing the power P.sub.2 at a frequency f.sub.1 shown in FIG. 9. Of course, the load power P.sub..delta. supplied to the load 6 is given by P.sub..delta. =P.sub.1 +P.sub.2.
As can be understood from the above description, by imparting the frequency-versus-characteristic such as mentioned above to the inverter 1 by providing the inverter control circuit constituted by the circuit elements 10 to 15 shown in FIG. 8, the inverter 1 and the AC power source 2 can share the power supply to the load 6 in accordance with the respective power-frequency characteristics.
However, in the known power supply system described above, no consideration is paid to the transient operation state which prevails immediately after the parallel operation has been validated by closing both the switches 3 and 4. Consequently, there arise various problems during a period in which no synchronism is established between the AC powers P.sub.1 and P.sub.2 of the two power sources, i.e., the inverter 1 and the AC power source or generator 2.
With the scheme of the known parallel operation control for the power supply system as described above, the power supply to the load 6 can certainly be shared by the inverter or power converter 1 and the power source 2 in the steady operation state. However, in the transient state, immediately after the system has entered the parallel operation, a large cross current will flow until synchronism is established between the two power sources 1 and 2. Accordingly, in-the system where the inverter 1 and the AC power source 2 are to be changed over without interrupting the power supply to the load 6, the inverter 1 and the AC power 2 are operated in parallel only for a short time. However, in case a large cross current flow which exceeds the rated current of the inverter 1 or the AC power 2 occurs during the parallel operation, the AC voltage V.sub.1 or V.sub.2 of the inverter 1 or the AC power source 2 droops, as a result of which the voltage of the AC bus 5 is lowered during the change-over period or immediately after the lapse of that period, giving rise to a problem that reliability of the power supply system is degraded. Additionally, even in the isolated operation succeeding to the change-over through the parallel operation, the output frequency of the inverter 1 droops in dependence on the effective power P, which in turn means that the output frequency decreases in dependence on the load 6, to another disadvantage.
Furthermore, when the phase of the AC voltage V.sub.2 of the AC power source 2 lags that of the AC voltage V.sub.1 of the inverter 1 upon entering the parallel operation, the AC power P.sub.2 from the AC power source 2 will be transferred toward the AC output terminals of the inverter 1. Consequently, an AC power P.sub.2 is regenerated at the DC input terminals of the inverter 1, resulting in that the DC voltage V.sub.d rises up, whereby there arises the possibility of the elements constituting the inverter 1 being damaged, to a further disadvantage.
It should be noted out that because the effective power P is detected by the power sensor 15, as shown in FIG. 8, the structure of the feedback control system becomes complicated, providing another cause for the deterioration of the reliability of the power supply system.